May 16, 2013
Understanding the mechanical response of super-stiff supramolecular nanotubes
Supramolecular assemblies – complexes of molecules joined by weak, reversible chemical bonds – offer promise for the creation of new materials with tunable properties. However, little is known about many of the assemblies' mechanical properties.
To gather more information, researchers at Northwestern University and University of California–Berkeley have teamed up to study the mechanical properties of cyclic peptide nanotubes (CPNs), supramolecular assemblies of small, ring-shaped, synthetic protein building blocks that self-assemble into rigid, macroscopic length nanotubes via inter-ring hydrogen bonding.
CPNs could potentially be used in nanocomposites to control porosity and improve mechanical properties such as strength and toughness, and they provide a potential alternative to inorganic nanotubes with the advantage of being dynamic structures. However, tailoring CPNs' properties for many of these applications requires precise understanding of their elastic behavior and susceptibility to fragmentation.
The dynamic character of supramolecular assemblies, arising from the weak nature of the stabilizing forces under thermal fluctuations, poses an important challenge on the assessment of their mechanical response using classical engineering analysis tools.
In this study, the bending rigidity and dynamical fragmentation of CPNs are quantified for the first time, establishing the theoretical basis to generate rectilinear structures with controlled aspect ratio and rigidity, which is of paramount significance for the generation of novel nanocomposites and nanoporous thin films.
Molecular dynamics simulations of CPN under mechanical load (Figure 1a), together with TEM image analysis (Figure 1b), have been employed to characterize persistence length (rigidity) of CPNs, showing good agreement between simulations (0.46 µm) and experiments (0.6 µm). This result suggests that CPNs are exceptionally stiff despite their small diameter, and hence may be suitable as rectilinear pores in nanoporous membranes.
The role of dynamic loading on the fragmentation and localization of the failure along the nanotube was also studied using a theoretical framework and numerical simulations. An exponential dependence of force and failure times was observed, with higher loads leading to stiffer responses and well defined failure locations (Figure 1c).
This suggests that the level and rate of the applied load can be tuned to control the polydispersity and length of the CPNs through a fragmentation process. These findings pave the way for creating new nanostructured materials from CPNs, such as CPN reinforced structural nanocomposites and subnanometer porous thin film membranes.
The researchers presented their work in the journal Nanotechnology.
About the author
Sinan Keten is Assistant Professor of Mechanical Engineering and Civil and Environmental Engineering at Northwestern University.